What are acceptable methods of calculating business mileage?
IRS Publication 463 says that, in lieu of a full mileage log, you can measure your total mileage for the year and calculate the percent of your miles that are business use, based on 1 logged week for each month. It implies, also, that similar methods are acceptable -- one website I saw suggested logging a 90-day period, and extrapolating your business-ersonal ratio from there.
My question: what is the standard (if any) for extrapolating business miles? For instance, if I logged mileage in detail for this month, December, could I use that log to justify claiming a comparable amount of business miles over the entire year (or in my case, the past 5 months), based on percentage of total mileage?
What about creating a full mileage log based on Google Maps measurements? Could I submit that along with a real 1-month log, demonstrating that they are functionally identical, to prove that the total mileage reimbursement I claim is accurate?
(For background: I'm a self-employed freelance writer/editor/consultant. Since August I've been using my vehicle for business, traveling 3-4 days a week from my home office to the office of my main client, some 40 miles away. I've kept receipts from parking and public transit, so there's proof I've been traveling, but I haven't kept a mileage log until now.)
Thanks very much!